Head Injury Clinical Assessment

4. Pupil response

The light reflex (page 138) tests optic (II) and oculomotor (III) nerve function. Although

II nerve dysfunction after head injury is important to record and may result in permanent visual impairment, it is the III nerve function which is the most useful indicator of an expanding intracranial lesion. Herniation of the medial temporal lobe through the tentorial hiatus directly damages the III nerve resulting in pupil dilatation with impaired or absent reaction to light. The pupil dilates on the side of the expanding lesion and is an important localising sign. With a further increase in intracranial pressure, bilateral III nerve palsies may occur. 0

J Space-occupying mass causing tentorial herniation presents with a III nerve palsy

Posterior communicating artery

III nerve

III nerve

Carotid arteries

Kernohan's -notch

(see below)

Posterior communicating artery

III nerve

III nerve

Carotid arteries

5. Limb weakness

Determine limb weakness by comparing the response in each limb to painful stimuli (page 30). Hemiparesis or hemiplegia usually occurs in the limbs contralateral to the side of the lesion but may also occur in the ipsilateral limbs. This is due to indentation of the contralateral cerebral peduncle by the edge of the tentorium cerebelli (Kernohan's notch). Limb deficits are therefore of limited value in localising the site of the lesion.

Descending pyramidal tracts

Decussation -

Contralateral limb weakness

Descending pyramidal tracts

Decussation -

Contralateral limb weakness

Points of possible damage

Ipsilateral limb weakness (false localising sign)

Points of possible damage

Ipsilateral limb weakness (false localising sign)

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